US2505465A

US2505465A - Surge protected alternating-current vacuum tube meter

Info

Publication number: US2505465A
Application number: US666910A
Authority: US
Inventors: Deketh Jan
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1942-02-25
Filing date: 1946-05-03
Publication date: 1950-04-25
Anticipated expiration: 1967-04-25

Description

April 1950 J. DEKETH 2505,465

SURGE PROTECTED ALTERNATING-CURRENT VACUUM TUBE METER Filed May 3, 1946 AGE/V said two electrodes,;as a result of which "cathode is deteriorated and many drawbacks Patented Apr. 25, 1950- SURGE PROTECTED ALTERNATING- CURRENT VACUUM TUBE METER Jan Delreth, Eindhoven, Netherlands, assignor, by .mesne assignments, to Hartford National Bank and lllrust Company, Hartford, Conn, as trustee Application May 3, 1946, Serial No. 666,910 In the Netherlands February 25, E942 Section 1, Public Law 690, August 8,1946 Patent expires February 25, 1962 6 Claims. (C1. '171--95) For measuring alternating voltages in which a small amount of energy onlymay be derived from the source of supply, use is frequently made of a vacuum. tube voltmeter comprising an amplifying tube arranged as an anode detector, the voltage to be measured being supplied to the control grid of the tube. In'practice such vacuum tube voltmeters are sometimes also used for measuring direct voltages.

In such tube voltmeters the control grid bias voltage required, which has a comparatively high value, is usually derived at least in part from a resistance-included in'the cathode lead of'the tube, the alternating voltage to be measured being applied between the control grid and that extremity of the cathode resistance which is remote from the cathode. The said cathode resistance which thus forms part of the input circuit of the tube voltmeter must then be bridged by a condenser having acapacity such that the R. 0.- time of the parallel circuit is great relative to the duration of the-period of the lowest frequency to be measured. Use may be made, for example, of a condenser-of about 8 microfarads.

A conventional tube voltmeter of the type de scribed'has the drawback of being unsuitable for measuring comparatively high alternating voltages, for example alternating voltages having a maximum instantaneous value of 150 volts or higher.

According to the invention, the said drawback in such tube voltmeters is avoided by including a high-ohmic resistance of, 'for example, 100,GQO

to 500,006 ohms in the input circuit or" the tube voltmeter andin series withthe condenser which bridges the cathode resistance of the tube.

The invention is based onthe recognition. of the fact that in the conventional tube voltmeters, immediately after a'high alternating voltage to be measured has been applied at the input terminals, the control grid during a short time periodically acquiresso high a positive potential rel ative to the cathode that a very high current or even breakdown phenomena occur between the the are involved.

By the use of a high-ohmic resistance connected-in accordance with'the invention, excessive currents and breakdown phenomenaend hence deterioration or the cathode are avoided, since new grid. current which always begins to priorto' the occurrence of the excess.

it breakdown phenomena brings about burr ' Such a voltage drop across'the high-ohmic resist of a vacuum tube voltmeter according 2 ance that the grid can atmost exhibit a small positive potential relative to the cathode, which cannot do any harm.

The high-ohmic resistance may be included,

for'example, in the conductor which connects the control grid of the tube to the input terminal concerned. In carrying out measurements this high-ohmic resistance, together with the grid-cathode capacity of the tube acts; however, as a voltage divider so that with high frequencies, at which the grid-cathode capacityhas s small reactance, a considerable attenuation of the signal to be measured occurs which is, in addition, variable with frequency. In view thereof the high-ohmic resistance is preferablylconnected in series with. the lay-pass condenser, the series-connection. thus obtained being in parallel to the cathode resistance of the tube.

After the measuring voltage has been applied, the bridging condenser of the cathode resistance after expiration of a period of time determined by the resistance and the capacity of the input circuit will be charged approximately up to the voltage corresponding to the measuring voltage.

' As a matter of fact, after expiration of this period, excessive currents or breakdown phenomena between the control grid and the cathode cannot occur. Since in this case the high-ohmic resistance may be dispensed with and the latter brings about a measuring error which varies with frequency, the high-ohmic resistance is preferably switched oif some time after the application of the measuring voltage and prior to carrying out the measurement proper, this switching out being effected, for example, with the aid of a resilient short-circuit switch which is open in the position of rest. With very low measuring voltage frequencies there occurs in general an additional condenser charging current and thus also a grid current when the resistance is shortcircuited. This grid current maybe limited, if desired, by intermittently short-circuiting the resistance.

The invention will be more clearly understood and readily carried into effect with reference to the accompanying drawing showing, by wayof example, one advantageous form of construction to the invention.

In the tube voltmeter in bridge connection wvhich is shown in the drawing the supply voltage is taken from a battery one branch of the bridge being constituted by resistances 2 and and the other by a triode A containing an indi rectly heated cathode anda cathode resistance 8 bridged by a paper condenser E. A sensitive ammeter l is included in the measuring diagonal of the bridge connection. The alternating voltage to be measured may be supplied to input terminals 8 which are connected directly to the control grid of the triode 4 and to a tapping point 9 of the resistance 3. The voltage drops produced at the resistance 6 and at the lower part of the resistance 3 which is located in the input circuit jointly provide the control-grid negative grid voltage for the triode. The voltages set up at the resistances 6 and 3 are of opposite directions and so chosen that the triode is biased approximately to cut off and operates as an anode detector. In operation, an alternating voltage applied to the control grid will give pulses of plate current on the positive half cycles and no current on the negative half cycles.

According to the invention, for the purpose of avoiding excessive currents between the control grid and the cathode of the triode 4 when a high alternating voltage to be measured is suddenly applied, the condenser has connected in series with it a high-ohmic protective resistance it which may be short-circuited before reading the meter 1, with the aid of a switch l2 which is normally open through the action of a spring II. To avoid erroneous readings, it is advantageous if the switch in the position of rest short circuits the measuring instrument.

It is evident that the closing of the switch ['2 some time (for example 2 to 4 secs.) after ap plying the alternating voltage to be measured may be eiifected automatically with th aid of means known per se, for example with the aid of a relay 13 included in the cathode or anode conductor of the triode. The invention is, of course, also applicable to vacuum tube voltmeters in which a tube having a plurality of grids is used instead of the triode 4.

What I claim is:

1. A vacuum tube arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a bypass capacitance, a protective resistance, said capacitance being connected in series with said protective resistance across said bias resistance, means to apply a voltage to be measured to said grid and through said bias resistance to said cathode, said bias resistance having a value at which said tube operates as an anode detector, said protective resistance having a value such that upon the application of said voltage to be measured and prior to th attainment of a charge on said capacitance a safety bias is developed across said protective resistance, and direct-current iiidicating means coupled to the output of said anode detector.

2, A vacuum tube voltmeter arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a by pass capacitance connected across said bias resistance, a sorce of anode potential connected to said anode and through said bias resistance to said cathode, means to apply a voltage to be measured to said grid and through said bias resistance to said cathode, said bias resistance having a value at which said tube operates as an anode detector, direct-current indicating means coupled to the output of said anode detector, a

protective resistance, and switching means for connecting said protective resistance in series with said capacitance, said protective resistance having a value such that upon the application of a voltage to be measured and prior to the at- 4 tainment of a charg on said capacitance a safety bias is developed across said protective resistance.

3. A vacuum tube voltmeter arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a bypass capacitance, a protective resistance, said capacitance being connected in series with said protective resistance across said bias resistance, a source of anode potential connected to said anode and through said bias resistance to said cathode, means to apply a voltage to be measured to said grid and through said bias resistance to said cathode, said bias resistance having a value at which said tube functions as an anode detector, said protective resistance having a value at which upon the application of a voltage to be measured and prior to the attainment of a charge on said capacitance a safety bias is developed across said protective resistance, a direct current indicator coupled to the output of said anode detector, and switching means for selectively in a first position shorting said indicator and in a second position shorting said protective resistance.

l. A vacuum tube voltmeter arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a bypass capacitance, a protective resistance, said capacitance being connected in series with said protective resistance across said bias resistance, an electromagnetic relay, a source of anode potential connected through said relay to said anode and through said bias resistance to said cathode, means to apply a voltage to be measured to said grid and through said bias resistance to sai cathode, said bias resistance having a value at which said tube functions as an anode detector, a direct-current indicator coupled to the output of said anode detector, and switching means for selectively in a first posi ion shorting said indicator and in a said position shorting said protective resistance, said switching means being maintained normally in said first position, said relay being arranged to actuate said switching means after a predeterm ned time interval.

5. A vacuum tube voltmeter arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a bypass capacitance, a protective resistance, said capacitance being connected in series with said protective resistance across said bias resistance, a source of anode potential connected to said anode and through said bias resistance to said cathode, means to apply a voltage to be measured to said grid and through said bias resistance to said cathode, said bias resistance having a value at which said tube functions as an anode detector, a first and second resistance connected in series across said potential source, a direct-current indicator connected between the junction of said first and second resistance and said cathode, said first and second resistances having values such as to constitute with said bias resistance and said tube a bridge circuit, and switching means for selectively in a first position" shorting said indicator and in a second position shorting said protective resistance.

6. A vacuum tube voltmeter arrangement comprising a vacuum tube having a cathode, a control grid and an anode, a bias resistance, a bypass capacitance, a protective resistance, said capacitance being connected in series with said protective resistance across said bias resistance, an electromagnetic relay, a source of anode potential connected through said relay to said anode and through said bias resistance to said cathode, said bias resistance having a value at which said tube functions as an anode detector, at first resistance, a second resistance having an adjustable tap, said first and second resistances being connected across said potential source and constituting with said tube and said bias resistance a bridge circuit, means to apply a voltage to be measured between said grid and said adjustable tap, a direct-current indicator connected between the junction of said first and second resistances and said cathode, and switching means for selectively in a first position shorting said indicator and in a second position shorting said protective resistance, said switching means being maintained normally in said first position, said relay being arranged to actuate 10 Number J AN DEKEI'H.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Williams, Jr Jan. 23, 1945 OTHER REFERENCES Vacuum Tube Voltmeters, Rider (19 1, J. F. Rider Publisher, Inc.), Figures 5-6, page '77, and Figures 7-3, page 95.